Signal communication privacy is a matter of concern in radiotelephone systems. Implementation of privacy techniques involving enciphering of signals and the processing of signals to place them within the transmission bandwidth of a determined transmission medium, such as a telephone system voice channel, usually are accompanied by signal bandwidth expansion. The inconsistent requirements of enlarged bandwidth due to signal processing and/or enciphering and limited bandwidth of a transmission channel can result in reduced quality of transmitted signals. In radiotelephone systems, the need for private transmission is particularly acute, and the bandwidth conflict is significant because the radio link in a communication path of such a system is frequently in series with a relatively narrow band channel of a wire circuit in the public switched telephone network.
Sample masking is one signal enciphering technique that is experiencing growing popularity because the enciphered signals lack remanent clear information energy and so are unintelligible. In sample masking, signal samples are combined by modulo arithmetic with a sequence of signals representing randomly, and with substantially equal probability, the discrete levels of an amplitude range in which the signal samples are quantized. However, the outlined enciphering process expands the bandwidth, as noted above, of a voice signal of reasonable quality beyond the bandwidth of a typical telephone channel. Attempts to meet that challenge at a given effective bit rate have resulted in arrangements that either tolerate substantial reduction in the quality of a transmitted signal, or require special transmission channels of adequate bandwidth, or are so expensive to implement that they are not practical for commercial purposes.
S. B. Weinstein teaches a sample masking system in a paper "Sampling-based Techniques for Voice Scrambling," ICC '80 Conference Record, Vol. 1, pages 16.2.1-16.2.6. Input voice samples are constrained to an amplitude range which is reduced at each extreme by an amount corresponding to two or three standard deviations of the ambient noise to make the noise in recovered speech the same as if no scrambling had been performed. Those same input signals are also band limited to a 2500 Hertz bandwidth so that, after the enciphering bandwidth expansion, the signals will have a bandwidth of about 2933 Hertz that approximately matches that of the channel to be used.
An R. T. Adams et al. U.S. Pat. No. 4,283,602 teaches a baseband transmission system of the sample masking type in which enciphered signals are low pass filtered to the same bandwidth as both the original input signals and the transmission medium which is to receive the enciphered signals. Experience has shown that such arrangements, employing only low pass filtering to a common bandwidth create intolerable intersymbol interference which renders the signal unintelligible.
In U.S. Pat. No. 4,398,062 to D. D. McRae et al. is shown another system of the sample masking type and employing low pass filters in the respective channels of a quadrature modulator to "condition the pulse trains" provided from the output of a circuit that splits the enciphered signal into two half-pulse-rate symbol trains. The nature of the conditioning is not explained.
A paper "An Efficient Technique for Sample-Masked Voice Transmission" by R. J. Cosentino et al. appeared in the IEEE Journal On Selected Areas in Communications, Vol SAC-2 No. 3 May 1984 at pages 426-433, and describes the use of linear filtering of an enciphered and sample masked signal to modify the enciphered component without processing the signal component. A so-called spectrum-modifier filter was approximated by truncating a weighted sin x/x filter to a length of 599 which is expensive to implement because of the length of the filter.
Another Cosentino et al. paper "Secure Voice-Bandwidth Modem" was presented at the 1982 Carnahan Conference on Security Technology, Lexington, Ky., May 12, 1982; and describes a modem employing single-channel sample masking with sample rate decimation and interpolation.
In Chapter 7 of Digital Communications: Microwave Applications, by K. Feher, Prentice-Hall, Inc., 1981, a paper "Correlative (Partial Response) Techniques and Applications to Digital Radio Systems" by A. Lender, includes discussion of the concept of introducing into a data signal a controlled amount of intersymbol interference so that a somewhat higher symbol rate can be accommodated in a channel of given bandwidth. Both binary and non-binary data signals are considered. Modulo addition precoding is indicated to reduce error propagation. Examples of modulo Q=4 (seven level) multilevel systems are presented. A 31-level system is mentioned but not disclosed. Bit slicing is used for data detection and is contemplated for error detection also. In data transmission systems, the noise effects injected during transmission usually do not exceed the amplitude span between adjacent levels.